The present invention relates generally to implantable medical interventional devices and methods for treating cardiac rhythm disorders, and more particularly to an implantable defibrillator for ventricular defibrillation, with pacing and sensing of the atrium and related methods of therapy using such implantable defibrillators.
Current implantable defibrillators perform a variety of functions designed to treat ventricular arrhythmias, including sensing of ventricular signals, detection of ventricular arrhythmias consisting of bradycardia, tachycardia, and fibrillation, and delivery of appropriate therapy automatically selected from among bradycardia and antitachycardia pacing, cardioverting and defibrillating shocks of the ventricles to correct the disorder. A serious problem with these devices is that a significant percentage of the defibrillating shocks delivered to the ventriclesxe2x80x94about 25%xe2x80x94are falsely fired, delivered while the patient is fully conscious. The statistic is supported by recordings of cardiac activity among patients whose implanted devices have Holter function capabilities, and study of the recorded time period immediately before and up to delivery of the defibrillating or cardioverting shock, as well as by numerous interviews of defibrillator patients. Aside from the extreme pain suffered from a false shock, the patient tends to quickly lose confidence in the reliability of the implant as a life-saving device.
A large part of the reason for the false shocking is that many patients develop atrial fibrillation and atrial flutter spontaneously, and, with a tendency for fast conduction through the atrioventricular (AV) node, the ventricle is driven at a high rate. If the ECG criteria for ventricular tachycardia or fibrillation on which the implanted device relies for performing its therapy functions are fulfilled, a high energy cardioverting or defibrillating shock will be delivered to the ventricle. The shockxe2x80x94albeit falsexe2x80x94is a proper response, given the criteria from which the determination was made. Rather, it is the data on which this response is based that is insufficient.
The solution to this problem of intermittent atrial fibrillation and flutter that can give rise to false shocks is to give greater attention to the status of the atrium. Currently available implantable defibrillator devices are unable to provide the solution because their focus is on the status of the ventricle. Recognition of atrial activity together with that of ventricular activity enables better discrimination of sinus rhythm, sinus tachycardia, ventricular fibrillation and ventricular flutter from one another. The better discrimination of the dysrhythmiaxe2x80x94or absence thereofxe2x80x94allows the device to more properly respond with a corrective therapy that is based on the true condition of the patient. In other words, the device can better distinguish which heart chamber is attributable to the arrhythmia, so as to respond in kind.
It is a principal aim of the present invention to provide an implantable defibrillator that monitors the atrial status as well as the ventricular status, to discriminate arrhythmias of atrial origin from arrhythmias of ventricular origin, from which to better select the proper electrical therapy to be delivered to the patient""s heart, and more specifically, to eliminate or at least substantially lessen the likelihood of false shocking.
Another problem which is not solved by the currently available spate of implantable defibrillators is the prominence of atrial arrhythmias which occur in implant patients because of a failure to address the atrial chamber. For example, the current devices perform ventricular pacing, but if retrograde conduction occurs the patient has a relatively high riskxe2x80x9440% or morexe2x80x94of developing atrial fibrillation. In contrast, patients who are experiencing constant atrial stimulation along with the ventricular pacing have a much lower riskxe2x80x94on the order of 5 to 10%xe2x80x94of developing intermittent or chronic atrial fibrillation.
Accordingly, another aim of the present invention is to provide an implantable defibrillator that performs pacing of the atrium as well as the ventricle, so as to enable better prevention of atrial arrhythmias.
According to one aspect of the present invention, an implantable defibrillator possesses the usual capability of ventricular defibrillation along with ventricular bradycardia and tachycardia pacing, and sensing of the ventrcular signals (i.e., ECG or cardiac signals) for determination of which of those therapies is to be delivered, but also performs stimulation of the atrium. Specifically, the device has the capability to pace the atrium to assure a constant or continuous rate of depolarizations, e.g., whether spontaneous (intrinsic, i.e., triggered by electrical activity of the sinoatrial (SA) node) or paced (i.e., stimulated, in the absence of such intrinsic activity, by operation of the implanted device). This type of atrial pacing assures that AV synchrony will be maintained, i.e., ventricular depolarizations are continuously synchronous with atrial depolarizations as a consequence of ongoing depolarizations of the atrium at the specified rate, with each atrial beat followed sequentially by a ventricular beat, under conditions in which the device is not called on to provide other therapies of a priority hierarchy that necessitate a different stimulation of the atrium such as antitachycardia pacing or cardioversion or defibrillation.
This fallback or xe2x80x9cdefaultxe2x80x9d condition of continuous stimulation of the atrium at a fixed pacing rate by the implanted defibrillator device serves to significantly reduce the incidence of atrial arrhythmias, and can also reduce or even eliminate dependence of the patient on prescribed antiarrhythmic medications or beta-blockers. Further, the assured synchronization of the atrial and ventricular contractions of the heart represents a hemodynamic improvement for many patients who are candidates for an implantable defibrillator, by which the overall cardiac performance of these patients is improved to an extent that additionally aids in reducing the occurrence of dysrhythmias.
In addition to pacing the atrium at a fixed rate which is appropriate for the particular patient who is to receive the implant, the defibrillator device is provided with a capability to sense the atrial rhythm, i.e., the atrial signal, independently of the ventricular signal. By doing so, and applying appropriate algorithms which compare the atrial and ventricular status, the implanted device can provide a more precise diagnosis of the nature of the underlying rhythm disorder. For example, if ongoing ventricular tachycardia is detected by the implanted device, the presence of normal sinus rhythm at the atrial sense signal input facilitates a diagnosis that the tachycardia is of ventricular origin. On the other hand, if the device senses ventricular tachycardia while the atrial sense signal reveals atrial flutter or atrial fibrillation, the origin of the rhythm disorder is determined to be in the atrium with a fast ventricular response.
Since both atrial and ventricular pacing are employed, as well as atrial and ventricular sensing, the implantable device of the invention effectively combines the advantages of DDD pacing with a conventional xe2x80x9cfull functionxe2x80x9d defibrillator, which as noted above, generally includes brachycardia and tachycardia pacing of the ventricle and cardioversion and defibrillation of the ventricle, DDD, of course, is part of the three-position ICHD (Inter-Society Commission on Heart Disease Resources) device code which indicates that the device is adapted to provide dual chamber pacing, dual chamber sensing, and both triggered and inhibited modes of response (atrial triggered and ventricular inhibited).
It is also desirable to provide the device with a rate adaptive or rate responsive capability which enables it to recognize whether the patient is engaged is resting or engaged in exercise, which is then used to adjust the rate according to the nature and extent of the exercise, and can also be taken into account in diagnosing whether a rhythm disorder is present (for example, in assessing whether a tachycardia is physiologic or pathologic). In a preferred embodiment, the invention employs a two-dimensional accelerometer as a sensor of activity and position of the patient. Thus, the DDD pacemaker with which the defibrillator is combined becomes a DDD-R (the xe2x80x9cRxe2x80x9d suffix being indicative of rate adaptive capability in the ICHD device code).
Atrial monitoring, detection and treatment which are effective to terminate an arrhythmia have the added benefits of lower energy dissipation and greater likelihood that treatment will be administered while the patient is conscious (with consequent easing of the task of successful treatment), compared to treating arrhythmias of ventricular origin. For example, atrial flutter is broken by rapid atrial stimulation and atrial fibrillation is terminated by applying a defibrillating shock to the atriumxe2x80x94to synchronize the atriumxe2x80x94using virtually the same antitachycardia or defibrillator subsystem as that for treating ventricular tachycardia and fibrillation, except that the energy requirements are significantly lower and can be tolerated by the conscious patient without significant pain.
According to another aspect of the invention, further improvements in diagnosis and treatment are obtained in a preferred embodiment by the use of fuzzy logic, which examines the extent to which a particular finding is true or false, allowing the decision on appropriate therapy to be made without regard to non-linearity of the findings. Determinations and selections are made according to the degree of membership of a particular statement (a finding) has to a certain class (e.g., the extent of truth or falsity of the finding).
Another feature of the invention is that the number of leads to be implanted for use with the device is reduced, with attendant simplification of surgical procedure and reduction of cost, because the same lead may be used for atrial and ventricular pacing, sensing and defibrillation. Two transvenous leads having a size of about 6xc2xd French may be employed, with pacing/sensing cathodal tip and sensing/pacing/shocking anodal ring for conventional bipolar pacing and sensing, and low polarization electrodes for shocking to allow intrinsic rhythm to be detected without masking by polarization currents.
Other aims of the invention, then, are to provide an implantable device that combines the capability for ventricular pacing, sensing and defibrillation with a DDD or DDD-R pacing capability to improve detection, diagnosis, and treatment of arrhythmias, including origin as being atrial or ventricular; provision of rate adaptive capability for both pacing rate adjustment according to activity and improved diagnosis of arrhythmias; use of fuzzy logic to simplify and enhance diagnosis and treatment; and capability to use fewer leads for device implantation.
In U.S. Pat. No. 5,243,980, an automatic cardioverter/defibrillator (ACD) is disclosed as having the capability to discriminate ventricular tachycardias from supraventricular tachycardias, and to distinguish sinus tachycardias from non-sinus tachycardias. The device electrically stimulates fat pads associated with the SA node and AV node, as part of the nervous system that regulates the rhythm of the heart. The device of the ""980 patent detects a ventricular tachycardia and then stimulates the nodal fat pads in synchronism with detected atrial depolarizations and/or ventricular depolarizations. The orgin of a tachyarrhythmia is determined from an observation of which fat pad, when stimulated, induces a predetermined change in the cardiac rhythm. If no change in the ventricular rate is observed upon stimulation of either fat pad, the ventricle is deemed the origin; whereas if the atrial rate or the ventricular rate decreases, depending on which fat pad is stimulated, the tachyarrhythmia is deemed to be supraventricular in origin, or a sinus tachycardia. Although the ""980 patent describes an implantable pacemaker/cardioverter/defibrillator including possible DDD pacing, neither the type of pacing nor the identification of rhythm disorders corresponds to that of the present invention. Rather, the fat pad stimulation technique is employed.